United were 3-0 down inside 20 minutes for the first time in the Premier League era, and it stayed that way as an Alexis Sanchez double and Mesut Ozil’s goal took Arsenal into second on goal difference.
And a shellshocked Van Gaal did not mince his words in the aftermath of the game as he promised to get to the bottom of his side’s awful start.
“When you start like that you can never win a game,” the United boss told Sky Sports. “We lost every duel, every second ball, we were not tight on their midfielders – and they can play.
“It’s a fantastic team – I said that in advance. When you give them that kind of space it’s difficult. After five minutes it’s already 2-0, and then it’s more difficult.
“I’m looking for [answers] still because I never have seen that of my team, so that is also the question that I put to my players. You cannot solve it at once because there is a lot of emotion, and I am emotional. You have to see the images also, but I know for sure it is like how I said.
“It was not only one player – then I can take a measure. No, it was more than half of my team, I think, that was not doing what we have agreed in our gameplan.
“You don’t expect it – we are top of the league, we have won all our last four matches and we have to protect our position. It is possible against Arsenal – I believe in that.
“The way we started was so without emotion and that kind of stuff – you cannot expect that. It’s difficult to understand, and we have to look for the reasons.”
Van Gaal’s post-Emirates inquest will have to wait until after the international break, though, much to his obvious frustration.
He said: “The biggest concern is that the players are flying out from London to go to the national teams, so you cannot prepare for our match against Everton.
“Last international break we lost against Swansea [before it] but won against Liverpool [after it], so we can do it. There’s not any issue there, but today was a shock for me.”
Asked if it was United’s worst performance since he became manager, Van Gaal said:
“Yes, I think so because we were not able to give a normal performance and Arsenal was much better, much more aggressive.”